1. Field of the Invention (Technical Field)
The present invention relates to thermoluminescence dosimeters (TLDs) for radiation dosimetry.
2. Background Art
Current dosimetry methods employ TLDs that have materials such as copper, Mylar(copyright), tin, and/or plastics to filter the radiation energy. See, e.g., M. Moscovitch, et al., xe2x80x9cMixed Field Personnel Dosimetry Using a Nearly Tissue-Equivalent Multi-Element Thermoluminescence Dosemeterxe2x80x9d, Radiation Protection Dosimetry 34:1/4, pp. 145-148 (1990): and T.F.L. Daltro, et al., xe2x80x9cThermoluminescence Dosemeter for Equivalent Dose Assessment in Mixed Beta and Gamma Fieldxe2x80x9d, Radiation Protection Dosimetry 85:1-4, pp. 145-148 (1999). This radiation filition causes varying degrees of energy to be absorbed by the TLD material underneath the filter(s). These dosimeters are processed using machines such as the Model 8800PC reader manufactured by Bicron/NE of Solon, Ohio. K. J. Velbeck, et al., xe2x80x9cNext Generation Model 8800 Automatic TLD Readerxe2x80x9d, Radiation Protection Dosimetry 84:1-4, pp. 381-386 (1999). Dose calculation algorithms attempt to determine radiation type and energy based solely on the ratios of light output without regard to the light wavelength. See, e.g., E. W. Bradley, et al., xe2x80x9cHarshaw Dose Calculation Algorithmxe2x80x9d, Sandia National Laboratories Report (1994). Daltro, et al., supra, and Moscovitch, et al., supra. Additionally, the use of radiation filters creates an angular dependence problem that results in an underestimation of the dose equivalent when a worker is not directly facing the radiation source. L. F, Friedman, et al., xe2x80x9cAngular Dependence of the Harshaw 8800/8812 TLD Systemxe2x80x9d, Sandia National Laboratories Report (1991).
TLDs operate in the following manner. Electrons within a material normally exist at an energy level known as the valence band. Through some outside physical processes, such as heating or, in the present case, a radiation event, these electrons can be excited into another region known as the conduction band. The electrons are free to move around in the conduction band, but they will always attempt to return to their ground state. Since energy must be conserved in such a transition, the crystal emits light photons that escape the material. The light is the medium that is used to create the correlation between absorbed energy and dose equivalent for an individual. However, once the light is emitted, all information about the radiation event(s) is lost. Therefore, a method must be used to trap the electron(s) in place until the dosimeter is ready to be processed.
Between the valence and conduction bands exists an area known as the forbidden region. Electrons in a material cannot exist in this region unless they are trapped there by an impurity when they attempt to return to their ground state. After being trapped, return to ground then occurs when the TLD reader heats the TLD with hot nitrogen gas, which causes the emission of photons of particular wavelengths. For dosimetry purposes, these impurities are purposely added to the TLD crystal and are referred to as dopants. For example, the present dosimeter employed by Sandia National Laboratories uses LiF that is doped with magnesium and titanium to create electron capture sites within a forbidden energy region of the crystal. For LiF:Mg,Ti, this region exists between 2.076-3.550 eV and will correspond to a light wavelength emission of 350-600 nm during dosimeter processing by the reader. Since current methods use the entire wavelength of light from the LiF, they depend totally on the dose calculation algorithm to make determinations about radiation type and energy.
A TLD crystal using LiF doped with magnesium, copper, and phosphorus (LiF:Mg,Cu,P) has recently been developed that appears to provide higher sensitivity than TLDs using Li:Mg,Ti. M. Moscovitch, xe2x80x9cPersonnel Dosimetry Using LiF:Mg,Cu,Pxe2x80x9d, Radiation Protection Dosimetry 85:1-4, pp. 49-56 (1999); O. R. Perry. et al., xe2x80x9cLiF:Mg.Cu,P Based Environmental Dosemeter and Dose Calculation Algorithmxe2x80x9d, Radiation Protection Dosimetry 85:1-4, pp. 273-281 (1999). The device and method of the invention is equally applicable to these and all other present and future TLD crystal materials.
The present invention provides an improved thermoluminescence dosimetry method, TLD, and TLD lens drawer that allows dosimeters to more accurately determine what type of radiation and energy an individual or physical environment has been exposed to during a monitoring period. And, having the knowledge of these conditions, he or she will also be able to more accurately calculate the dose equivalent received by the worker or environment.
The present invention is of an improvement to existing thermoluminescence devices comprising a sensitive element comprising one or more thermoluminescence crystals, the improvement comprising a lens drawer comprising one or more bandpass filters. The bandpass filters are preferably equal in number to the crystals (such as four) and comprise lenses.
The invention is also of a thermoluminescence device lens drawer comprising one or more bandpass filters. In the preferred embodiment, the bandpass filters are equal in number to a number of thermoluminescence crystals of a corresponding thermoluminescence device (such as four. The bandpass filters preferably comprise lenses.
The invention is additionally of a thermoluminescence dosimetry method comprising: heating one or more thermoluminescence crystals; passing light from the one or more crystals through one or more bandpass filters; and detecting light passed through the one or more bandpass filters. In the preferred embodiment, the bandpass filters are located in a thermoluminescence device lens drawer are equal in number to the crystals (such as four), and comprise lenses.
Objects, advantages and novel features, and further scope of applicability of the present invention will be set forth in part in the detailed description to follow, taken in conjunction with the accompanying drawings, and in part will become apparent to those skilled in the art upon examination of the following, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.